[PATCH] userfaultfd: reset err when move_pages_ptes succeeded

[Foxie Flakey <foxieflakey@gmail.com>]

There is an edge case when move_pages_ptes fails with EAGAIN and then outer
loop attempt to retry and succeeded but "err" isn't reset. This leads to
outer loop retry erroneously on same set of pages that was successfully
moved. This is fixed by resetting "err" when move_pages_ptes succeeded.

Fixes: 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Signed-off-by: Foxie Flakey <foxieflakey@gmail.com>
Assisted-by: ChatGPT:GPT-5.6-Luna
---
 mm/userfaultfd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c3adedaaf7d5..595e7e232f90 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2069,10 +2069,12 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 			ret = move_pages_ptes(mm, dst_pmd, src_pmd,
 					      dst_vma, src_vma, dst_addr,
 					      src_addr, src_end - src_addr, mode);
-			if (ret < 0)
+			if (ret < 0) {
 				err = ret;
-			else
+			} else {
+				err = 0;
 				step_size = ret;
+			}
 		}

 		cond_resched();

base-commit: 62cc90241548d5570ee68e01aaba6506964e9811
-- 
2.55.0